There are a number of conventional devices available for delivering drugs to a patient. More specifically, completely implantable drug delivery systems are available. Examples include the ISOMED and SYNCHROMED implantable pumps available from Medtronic, Inc., Minneapolis, Minn. USA.
Completely implantable drug delivery systems typically include a pump which stores and infuses the drug in a desired infusion mode and rate, and a catheter which routes the drug from the infusion pump to the desired anatomic site. Implantable pumps may be large and are typically implanted in areas of the body with available volume that is not completely filled with body organs, such as the abdomen. The target site for drug infusion may, however, be located at a distance from the pump. A thin flexible catheter is typically implanted to provide a guided pathway for drugs from the pump to the target location.
Implantable pumps are often used to treat neurological diseases; examples are chronic pain and intractable spasticity. These conditions require treatment for a long time, frequently for the lifetime of the patient. An implantable pump can deliver drugs at a desired rate without intervention for a long time, and make drug therapy much easier and more accurate. Large doses of oral drugs would be required since the blood-brain barrier prevents most of the drug from reaching the central nervous system. Some of the drug that is blocked by the blood-brain barrier will instead travel to other organs, and can cause undesirable side effects. A catheter can, however, penetrate the membranes that comprise the blood-brain barrier and infuse the drug directly to the target receptors.
The neurological drug receptors for many therapies, such as pain and spasticity, are located in the spinal cord. A catheter cannot be surgically connected to the spinal cord because it could damage other neurons and cause serious neurological problems. The brain and spinal cord are surrounded by cerebrospinal fluid (CSF). CSF provides a cushioning effect for the spinal cord, but also provides a vehicle to deliver substances such as proteins, glucose, and ions (e.g. sodium, calcium, and potassium) to the central nervous system. Neurological drug infusion systems are designed to utilize this property of CSF. The drug is infused into CSF and then distributed through the CSF to the receptors in the spinal cord. These systems typically rely on infusion at one location.
Other limitations of known implantable systems and methods of drug delivery is that the systems may be limited to a single reservoir from which only one drug solution can be delivered at any given time. The density of the drug solutions delivered by such systems cannot be changed after implantation of the devices.